1. Field of the Invention
The present invention relates to an electron microscope equipped with an electron biprism for obtaining information about the thickness distribution across a sample, an electric field distribution possessed by a sample, a magnetic field distribution possessed by a sample, or other similar information.
2. Description of the Related Art
In the prior art transmission electron microscope, an electron beam is directed to a sample. A sample image consisting of an electron beam transmitted through the sample is magnified by a magnetic lens and projected onto a screen, thus permitting observation of the sample.
An electron microscope equipped with an electron biprism is included in such conventional transmission electron microscopes. In this electron microscope, the electron biprism produces carrier fringes owing to interference of the electron beam not transmitted through the sample with the electron beam transmitted through the sample. The carrier fringes overlap the image of the electrons transmitted through the sample, giving rise to a hologram. The thickness distribution across the sample, an electric field distribution possessed by the sample, a magnetic field distribution possessed by the sample, or other information is obtained by observing the hologram on a screen.
FIG. 2 schematically shows a conventional electron microscope equipped with such an electron biprism. FIGS. 3(a) and 3(b) illustrate the electron biprism. FIG. 3(a) indicates the state in which the electron biprism has been turned off, and FIG. 3(b) indicates the state in which the biprism has been turned on.
In FIGS. 2 and 3(a) and 3(b), an electron microscope 1 has an electron gun 3 emitting an electron beam 2. The electron beam emitted by the electron gun 3 is accelerated by an accelerating portion 4. An illumination system 5 directs the electron beam 2 to a sample 6 after appropriately converging or diverging the beam 2. An objective lens 7 focuses a sample image originating from the electron beam transmitted through the sample 6. An electron biprism 8 is used to create a hologram. A line electrode 8a consists of a conductive wire having a diameter of about 0.3 to 0.6 xcexcm, the wire being stretched perpendicular to the electron beam path. A voltage source 8b applies a voltage of tens to hundreds of volts, for example, to the line electrode 8a. A switch 8c is connected across the voltage source 8b. Grounding electrodes 8d and 8e extend parallel to the line electrode 8a and are located on the opposite sides of the line electrode 8a. The grounding electrodes 8d and 8e regulate the electric field produced around the line electrode 8a. A focusing system 9 includes the electron biprism 8. In this focusing system 9, a hologram created by the electron biprism 8 is magnified by plural stages of magnetic lenses 10 and focused onto a screen 11. A transmission electron image of the sample 6 is indicated by 12. The optical axis of the electron beam is indicated by O. The line electrode 8a produces a shadow S.
In the electron microscope 1 equipped with the electron biprism 8 constructed in this way, when the electron biprism has been turned off, the switch 8c is thrown to the ground side as shown in FIG. 3(a). Under this condition, the voltage of the voltage source 8b is not applied to the line electrode 8a; ground potential is applied to the line electrode 8a.
When the electron beam 2 emitted by the electron gun 3 hits the sample 6, the electron beam 2a not transmitted through the sample 6 and the electron beam 2b transmitted through the sample 6 are converged by the objective lens 7. Then, this converged beam 2 passes through the electron biprism 8 located under the objective lens 7. Since ground potential is given to the line electrode 8a, electrons 2 from the objective lens 7 are not deflected by the line electrode 8a during passage through the electron biprism 8.
Therefore, as shown in FIG. 3(a), the transmission electron image 12 of the sample 6 and the shadow S of the line electrode 8a are separated and projected onto the screen 11. At this time, the position of the sample 6 is adjusted and the angular position of the line electrode is adjusted by a rotary mechanism so that the transmission electron image of the sample 6 is formed only on one side of the shadow S of the line electrode.
On the other hand, when a hologram consisting of a TEM image of the sample 6 on which carrier fringes are superimposed is observed, the switch 8c is thrown to the side of the voltage source 8b as shown in FIG. 3(b) to turn on the electron biprism 8. Under this condition, a positive voltage from the voltage source 8b is applied to the line electrode 8a. Electron beam parts which are converged by the objective lens 7 and hit on the opposite sides of the line electrode 8a in the same way as in the foregoing process are attracted toward the center by the positive voltage from the voltage source 8b. The beam parts are made to overlap each other in a lower position, thus creating carrier fringes. These carrier fringes are made to overlap the TEM image 12, creating a hologram. The hologram is enlarged by the focusing system 9 and projected onto the screen 11. By observing this hologram, the thickness distribution across the sample 6, an electric field distribution possessed by the sample, or a magnetic field distribution possessed by the sample can be obtained as information about variations in phase of the electron beam.
The electron microscope equipped with the aforementioned electron biprism has an excellent feature that allows the user to take the distribution of electric or magnetic field possessed by the sample 6 as information about variations in phase of the electron beam 2. Where a magnetic field distribution possessed by a sample is examined, if it is a magnetic sample, then it is necessary to place the sample 6 in a space where the effects of magnetic field can be neglected.
In the above-described conventional electron microscope 1, the sample 6 is placed in a strong magnetic field set up by the objective lens 7 and so it is necessary to turn off the objective lens 7 in examining the magnetic field distribution possessed by the sample if it is magnetic in nature.
However, the objective lens 7 plays a role for forming a crossover point P of the electron beam in an appropriate position between the sample 6 and the electron biprism 8 as shown in FIGS. 3(a) and 3(b), i.e., plays a role for forming a light source P for the electron biprism 8. The objective lens 7 further serves to form a TEM image of the sample in an appropriate position behind the electron biprism 8. Therefore, if the objective lens 7 is turned off, it is difficult to appropriately set the focal points of the light source P and hologram relative to the biprism 8. In addition, it is impossible to vary the magnification of the TEM image of the sample on the hologram-focused plane.
On the other hand, the spacing of the carrier fringes can be varied by the voltage applied to the biprism line electrode 8a. If the spacing of the fringes is narrowed in order to enhance the resolution of phase information, the contrast of the carrier fringes deteriorates. As a result, the signal-to-noise ratio of the obtained phase information becomes impaired.
Accordingly, in the conventional electron microscope shown in FIG. 2, the ratio of the magnification of the TEM image of the sample in the objective image plane to the spacing of the interference fringes is required to be variable to obtain phase information at the highest signal-to-noise ratio under the desired resolution, for the following reason. If the spacing of the interference fringes is finely controlled, the image contrast can be enhanced by controlling the magnification of the TEM image of the sample in the objective image plane. However, if the objective lens 7 is turned off as mentioned previously, it is impossible to vary the magnification of the TEM image of the sample in the objective image plane.
It is an object of the present invention to provide an electron microscope equipped with an electron biprism capable of effectively extracting information about variations in phase of an electron beam and of enhancing the spatial resolution of the phase information further.
This object is achieved in accordance with the teachings of the present invention by an electron microscope equipped with an electron biprism for causing an electron beam transmitted through a space where there is no sample and an electron beam transmitted through a sample to overlap each other, thus creating carrier fringes. This microscope is characterized in that plural lenses are placed between the sample and the electron biprism.
Other objects and features of the invention will appear in the course of the description thereof, which follows.